1. Field of the Invention
The present invention relates to an apparatus to generate a surround signal. The present invention particularly relates to an apparatus, to generate a surround signal based on a difference component of a two-channel stereo signal, and is for example, a surround circuit, an audio device, a surround device and an acoustic field controller.
2. Description of the Related Art
Conventionally, a two-channel stereo system is known in which two separate left and right speakers, which produce different sounds, are arranged in front of an audience to produce a sound similar to a live sound. A surround device in which right and left speakers are additionally arranged behind an audience to produce a three-dimensional acoustic sound, to thereby enhance the live sound effect, is also known.
In a prior sound device, the sounds for two front speakers and two rear speakers are generated especially for four channels. The sound of each channel is produced by each speaker corresponding to the four channels. However, due to recent development of circuit technique, a surround device in which a four-channel output signal is generated from a two-channel stereo signal using a surround signal processing circuit and the four-channel output signal is reproduced by four speakers arranged in front of and behind the audience has been proposed. For example, see FIG. 4 of Kokai (Japanese Unexamined Patent Publication) No. 5-091600 and FIGS. 1 and 2 of Kokai No. 7-040760.
A two-channel stereo signal (a left channel signal component L and a right channel signal component R) is input into a surround signal processing circuit of the surround device. The surround signal processing circuit processes therein the two-channel stereo signal and generates and outputs four signals consisting of a channel signal component FL produced by a front left speaker, a channel signal component FR produced by a front right speaker, a channel signal component RL produced by a rear left speaker, and a channel signal component RR produced by a rear right speaker.
However, generally, in such a surround signal processing circuit, the four-channel surround signal is generated based on a difference component which is obtained by removing a correlation component of the stereo two-channel signals L and R, for example, (L-R) component. Consequently, if separation of the input two-channel stereo signals L, R varies for some reasons, for example, a change in the electric field intensity when receiving an FM radio wave, or multi-path noise, the (L-R) component is reduced, resulting in reduction of the volume of the surround signal. This causes the volume of the sound produced by the rear speakers to change or no sound is produced by the rear speakers, in the worst case.